We have measured the resistive transition of very thin superconducting Al films in an applied magnetic field $H$. Values of the critical field $H_c$ were obtained as a function of temperature $T$, film thickness, and field direction. Qualitative and quantitative comparisons are made here with the theory of high-field superconductors including the effects of Pauli paramagnetism, spin-orbit interaction, and thermodynamic fluctuations. In particular, the predicted first-order transition to the normal state at low temperature and large $H$ was observed. From theoretical analysis of the $H_c-\mathrm{vs}-T$ data, values of the spin-orbit scattering rate $b$ of about 0.2 were obtained. The thickness of the films studied were in the range 35-1000 Å, and critical fields up to 55 kOe were observed.

• Received 6 April 1973

DOI:https://doi.org/10.1103/PhysRevB.8.5098